The invention relates to a heat exchanger of the layered-type and an advantageous use thereof, and more particularly, to a layered-type heat exchanger having a plurality of fluidically parallel first layers which define first heat exchanger ducts through which a first medium is capable of flowing, and in each case a second layer between the two first layers, the respective second layer being in thermal contact with the two adjacent first layers and defining at least one second heat exchanger duct through which a second medium is capable of flowing. Such a heat exchanger can be employed, for example, as an evaporator in a reforming plant, in which a hydrocarbon or hydrocarbon derivative is steam reformed for the purpose of generating a hydrogen-rich gas. The hydrogen-rich gas can then serve, for example, for feeding fuel cells in a fuel-cell vehicle.
Various versions of such heat exchangers of the layered or plate-type are known. This type of heat exchanger typically has a construction consisting of plates, discs or films stacked one above the other, with the result that essentially plane layers are formed. An alternative construction consists of tubes lying one in the other, with the result that layers resembling cylindrical rings are formed.
For specific applications, multi-stage heat exchangers are required, in which two or more stages are fluidically arranged in series or in parallel with respect to one and/or the other medium, particularly also for an evaporator in a reforming plant. Thus, the German Patent document DE 44 26 692 C1 describes a two-stage evaporator for evaporating a reactant mass flow with the aid of a heat transfer medium, in which the two evaporator stages are fluidically connected in series with respect to the reactant mass flow and with respect to the heat transfer medium. In this case, each stage may, in turn, consist of two evaporator parts which are fluidically connected in parallel with respect to the reactant mass flow and/or to the heat transfer medium. In particular, this is used for the purpose of evaporating water and methanol in parallel evaporator parts, and for subsequently combining the two components to form the reactant mass flow. The evaporator has a construction consisting of films with, alternately, ducts for the heat transfer medium and ducts for the reactant mass flow. The two evaporators can be integrated into this construction in that, in a group of the two alternating film layers, the first stage is formed by a first duct which is directly connected to an inflow and the second stage is formed by ducts which emanate from the first duct which thereby constitutes a distributor duct.
The laid-open European publication EP 0 861 802 A2 discloses a reforming unit with an integrated evaporator of the layered type, in which the evaporator comprises an actual evaporation layer and a steam-superheating layer located fluidically downstream with respect to the medium to be evaporated. Heat, which is generated by a catalytic combustion operation, can be supplied to the two evaporator layers from adjacent heating layers.
The rapid cold-starting behavior customary in other motor vehicles is required for fuel-cell vehicles, thus necessitating a correspondingly rapid cold-starting behavior of the evaporator if a steam reforming plant is used for hydrogen generation. If, for this purpose, a small light-weight starting evaporator and a main evaporator are fluidically connected in series with respect to the medium to be evaporated, this results in an additional pressure loss through the starting evaporator and makes temperature regulation under normal operation more difficult. In an alternative solution the batch steam to be reformed is generated in a one-part evaporator and, depending on the operating situation, that is to say cold starting or normal operation, can be conducted through a steam valve in controlled fractions into a first and/or second reformer stage. This solution has the problem that, during cold starting, steam condenses on the initially cold valve and a comparatively complicated valve is necessary because of the extreme ambient conditions due to the hot batch steam.
The technical problem on which the invention is based is, therefore, to provide a heat exchanger of the above-mentioned type and an advantageous use thereof, which allows a different thermal control of the one medium controllably through a first or a second heat exchanger part or in controllable fractions in parallel in both respective heat exchanger parts.
The invention solves this problem by providing a layer-type heat exchanger having a plurality of fluidically parallel first layers which define first heat exchanger ducts through which a first medium is capable of flowing, and in each case a second layer between the two first layers, the respective second layer being in thermal contact with the two adjacent first layers and defining at least one second heat exchanger duct through which a second medium is capable of flowing. The respective second layer comprises two fluidically parallel layer plies, each of which defines an associated second heat exchanger duct. Means are provided for the controllable supply of the second medium to the one layer ply in each case and/or to the other layer ply in each case. This type of heat exchanger is used as an evaporator in a reforming plant for the evaporation of a batch to be reformed. The one layer plies form a starting evaporator part and the other layer plies form a main evaporator part.
In the heat exchanger according to the invention, which has a construction consisting of alternating layers through which flows a first or a second medium, the respective second layer is divided into two fluidically parallel layer plies. The respective two layer plies of the second layers are assigned means for supplying the corresponding second medium in each case to one layer ply and/or in each case to the other layer ply in controllable fractions. By means of the two layer plies, the respective medium can be thermally controlled differently in a desired manner and/or also divided in any desired way.
When this heat exchanger is used as an evaporator in a reforming plant, the batch to be reformed can be evaporated correspondingly in controllable fractions in one evaporator part or the other. This is done in order, for example, to supply two different reformer stages independently with the vaporous educt and/or to design one of the two evaporator parts, integrated into the layered construction, specially for the cold-starting situation, that is to say for low inertia and therefore rapid cold-starting evaporation, whereas the other evaporator part is designed for normal operation in the warmed-up state.
The layered construction according to the invention has the further advantage that the two layer plies of every second layer can be brought into thermal contact with a first medium in parallel, not in series. When the heat exchanger is used as an evaporator in the reforming plant, in such a way that the one layer plies of the second layer function as a main evaporator in normal warmed-up operation and the other layer plies function as a starting evaporator particularly for the cold-starting situation, the main evaporator can be heated directly by the first medium, without the latter first being conducted via the starting evaporator.
In a heat exchanger according to a preferred embodiment, different heat transmission capacities are provided for the respective two layer plies, for example by means of effective heat transmission surfaces of different size. When the heat exchanger is used as an evaporator, the one layer plies can thus be designed specially for the cold-starting situation and the other layer plies specially for warmed-up operation.
In a heat exchanger according to a preferred embodiment, the supply means for the second medium contain a control valve, by which the second medium can be apportioned in controllable fractions into the two fluidically parallel layer-ply groups. In an evaporator application, this has the particular advantage that the control valve is arranged on the liquid side of the second medium to be evaporated, not on the steam side, and therefore does not have to be designed for high steam temperatures. This also does away with the problem which impedes rapid cold starting, namely that of condensation of evaporated medium on a still cold steam-side valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.